Segment Information - Real Estate Investments, at Cost of Company's Reportable Segments (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Segment Reporting Information [Line Items]
Operating properties	$ 3,437,074	$ 3,474,109
Right of use asset - operating leases, net	20,478	20,977
Construction-in-progress	266,263	210,311
Land held for development	118,684	117,984
Prepaid leasehold interests in land held for development, net	27,762	39,185
Philadelphia CBD
Segment Reporting Information [Line Items]
Operating properties	1,430,724	1,433,927
Pennsylvania Suburbs
Segment Reporting Information [Line Items]
Operating properties	875,586	871,530
Austin, Texas
Segment Reporting Information [Line Items]
Operating properties	771,977	728,741
Metropolitan Washington, D.C.
Segment Reporting Information [Line Items]
Operating properties	271,929	352,794
Other
Segment Reporting Information [Line Items]
Operating properties	$ 86,858	$ 87,117